Hey,
To answer your question: I'm not too familiar with the M-15--one came up for sale here locally and I saw some pictures. It certainly looks nice. I would be inclined to worry about its age it wasn't very well taken care though...
I've actually used my friends' M-3500 for a little bit--I love it. I definitely preferred its EQ to the one on my M520. My friends who co-own the board aren't madly in love with the preamps. But overall I think it's a great board--tonnes of routing options, barograph meters plus VUs for the main stereo bus; oscillator etc.
You can download the manual here: Found! - Tascam Music Mixer
cheers,
Peter